1. Field of the Invention
This invention relates to endoscopes and more particularly to endoscopes adopted for sterilization by autoclaving.
2. Description of Related Art
Endoscopes come in two basic forms. In one form the endoscope is flexible. Optical fibers transfer an image from an optical objective to an eyepiece or other viewing device. The fibers pixelate the image from the optical objective. Endoscopes of the second form are called rigid endoscopes. This invention, is applicable to rigid endoscopes. A rigid endoscope includes a tubular structure that carries an optical objective at a distal end and a relay lens system that transfers the image to a viewing device or eyepiece at a proximal end. Such devices provide better spatial resolution than flexible endoscopes do.
Rigid endoscopes are used in various surgical and diagnostic-medical procedures, so it is important that devices be capable of being sterilized. There are different ways to sterilize medical instruments. However, autoclaving is a preferred process. During this process, an endoscope is placed in a autoclave and steam is introduced to gradually raise the temperature to about 270° F. This temperature is maintained for some interval and then reduced in a controlled manner.
During this process, temperature differentials exist throughout the endoscope. Endoscopes, particularly rigid endoscopes, typically comprise materials characterized by different thermal coefficients of expansion. Consequently, during the autoclaving process it is possible for an endoscope to be subjected to different mechanical stresses. Experience has shown that repeated autoclaving of rigid endoscopes can damage seals whereupon steam can enter the endoscope with resultant condensation in the optical path distorting the image.
One approach for minimizing thermally induced stresses in rigid endoscopes has been to select materials that withstand all the temperatures involved with autoclaving and that minimize the effects of thermal expansion. For example, in U.S. Pat. No. 5,599,278 to Hibbard a housing, eyepiece, insertion tube, windows and light pipe are comprised of materials which withstand a temperature of at least 1200° F., well above a typical autoclaving temperature. In addition, components are composed of materials having a low thermal expansion coefficient with high thermal conductivity and high thermal shock resistance.
U.S. Pat. No. 5,944,656 to Pollack et al. discloses an endoscope in which cylindrical spacers position various lenses along an optical path in a tubular housing. Proximal and distal windows seal the ends of the housing. The windows are brazed to the endoscope thereby eliminating any adhesives, epoxies or other organic materials. In one embodiment, the relay lens system includes spacers with slots so they can flex individually like a bellows. Consequently the spacers maintain compression on the lenses over a varying temperature range to ensure that they will not tilt when the endoscope is thermally stressed.
In addition endoscopes are often subject to mechanical shock as a result of mishandling. For example, it is not unusual for an endoscope to be dropped. The Pollock et al. patent introduces spacers with lateral slots. These spacers are stated to act as gimbal springs to allow the spacers to flex slightly like a bellows. The spacers maintain compression on the lenses over varying temperature range without canting or tilting during expansion or contraction. However, such spacers, while effective for absorbing long term stresses as introduced by autoclaving, do not readily absorb transient shocks, such as the shocks introduced when an endoscope strikes the floor or other object. The effect of such transient shocks would be the same regardless of whether the sleeves contain lateral slots.